In answer to "What is it" I think you will find its a chemical naturally found in the body which carries fuel into the muscles at the muscle end. I think your muscles also store creatine in them so it works two fold.

Firstly it is supposed to aid the fueling/recorvery process and secondly it should allow you to train longer/harder etc as the muscles can sustain exercise for longer. Both for obvious reasons.

HTH's.